Hydraulic drive systems for construction machines such as hydraulic excavators include those controlling the delivery flow rate of the hydraulic pump (main pump) so that the delivery pressure of the hydraulic pump becomes higher than the maximum load pressure of a plurality of actuators by a target differential pressure. This control is called “load sensing control”. In such a hydraulic drive system performing the load sensing control, the differential pressure across each of a plurality of flow control valves is kept at a prescribed differential pressure by use of a pressure compensating valve to make it possible during the combined operation (driving two or more actuators at the same time) to supply the hydraulic fluid to the actuators according to a ratio corresponding to the opening areas of the flow control valves irrespective of the magnitude of the load pressure of each actuator.
In such hydraulic drive systems performing the load sensing control, each pressure compensating valve is generally configured to fully close when the spool moving in the direction of decreasing the opening area reaches the stroke end, as described in Patent Literature 1, for example.
Meanwhile, Patent Literature 2 describes a hydraulic drive system that is configured so that each pressure compensating valve does not fully close even when the spool moving in the direction of decreasing the opening area reaches the stroke end.